Nacreous or pearlescent pigments are lamellar or plate-like pigments which impart a pearly or nacreous luster to objects on which or in which they are used. Natural pearl essence, a mixture of guanine and hypoxanthine obtained from the scales of fish, has long been used in cosmetic formulations. Synthetic nacreous pigments developed for cosmetic use include bismuth oxychloride, bismuth oxychloride-mica and titania coated mica. The bismuth oxychloride-mica powders have the advantage of softness, good compressibility and high luster.
Bismuth oxychloride-mica pigments are made by precipitating bismuth oxychloride crystals in the presence of mica. In general terms, this is accomplished by hydrolyzing a soluble bismuth compound in an aqueous slurry of mica. While there are a number of ways of accomplishing this, a preferred procedure is to first prepare an acidic slurry of wet ground mica to which is added a solution of a soluble bismuth salt. The bismuth compound hydrolyses to form a bismuth oxychloride precipitate. The acidity of the system is maintained by adding a solution of a base, such as sodium hydroxide, for neutralization of the acid formed by the hydrolysis reaction. The amount of the bismuth solution is controlled so as to obtain the desired ratio of BiOCl to mica, generally in the range of 1:4 to 4:1, preferably 1:1.5 to 1.5:1.
The pigment is concentrated from the aqueous slurry, for instance, by filtration, and is usually washed free of salt with water, ethanol, or the like.
The BiOCl-mica pigments are further processed to various types of finished products. In one case, the filter cake of pigment is dried to a powder with or without an added dispersing agent. BiOCl-mica, and indeed many nacreous pigments, are often processed to a paste or suspension. A wet form helps to keep the crystals dispersed and unagglomerated. In a typical case, the BiOCl-mica filter cake is flushed with an oil, such as castor oil or mineral oil. During this flushing operation, the pigment, originally wet with water, is transferred to pigment wet with oil and the water is expelled as a second phase which can be removed either by decanting or by drying under vacuum or by a combination of these techniques.
A flushing of BiOCl-mica pigments is complicated by the surface properties of the pigment. The acidic, aqueous slurry of BiOCl-mica pigment is usually neutralized to at least some extent in order to avoid equipment corrosion problems. If the neutralization is only carried out to a pH of 2.5-3.5 before filtering, the surface of the pigment is hydrophilic making the transfer of the pigment from the aqueous to the organic phase very difficult and often impossible. If the pH is raised to 4.0 or above, the surface of the pigment tends to be somewhat less hydrophilic and the transfer to the organic phase is clean. Unfortunately, some iron ions are present in the final pigment slurry due to the dissolution of iron from the mica during the precipitation of the bismuth oxychloride, and the iron tends to precipitate if the pH is raised above about 3.5. The precipitated iron hydroxide gives the pigment a yellow discoloration, which is objectionable for some uses.
A treatment has now been discovered which enhances the luster of the BiOCl-mica nacreous pigment. It has further been discovered that depending upon the amount of treating agent and the degree of neutralization, the transferability from aqueous to organic phase can be easily and cleanly accomplished. For example, in accordance with the present invention the pigment in the filter cake can be transferred to the organic phase even when the pH has only been increased to about 2.5-3.0. In other words, it is possible to obtain a good flush to the organic phase while keeping the near-white color of the pigment.
Accordingly, it is the object of the present invention to provide a process for treating BiOCl-mica nacreous pigments which will enhance the luster of the pearlescent material and also enhance the aqueous-organic phase transferability of the pigment at a sufficiently low pH to avoid the concurrent precipitation of discoloring iron hydroxide. This and other objects of the invention will become apparent to those skilled in the art from the following detailed description.